Power-synchronization control (PSC), also known as synchronous-machine emulating control or virtual synchronous machine, was originally invented for the control of an HVDC Light transmission where both terminals were situated in very weak ac networks. PSC is disclosed in EP2319170. The method is similar, but not identical, to the virtual synchronous machine. PSC can be used for any grid-connected voltage-source converter (VSC), not just HVDC.
The name PSC refers to how the method achieves synchronization to the grid; via the control of the active power. PSC therefore does not utilize a phase-locked loop (PLL). PSC is effective for connection to weak grids, with a short-circuit ratio (SCR) below 2. However, it gives a poorly damped closed-loop system for connection to a strong grid. In that case, the traditional method of vector current control (VCC) with outer loops, including a PLL, is preferable.
In a situation where the SCR is known to vary within a wide range, including SCR<2, there are currently two options.
1. Tune the PSC power-control gain on-line as a function of the SCR.
2. Make a switchover between PSC and VCC at a predetermined SCR.
Neither option is desirable. The preferred situation is to have a control system that is robust to the SCR.
LIDONG ZHANG ET AL: “Power-Synchronization Control of Grid-Connected Voltage-Source Converters”, IEEE TRANSACTIONS ON POWER SYSTEMS, IEEE SERVICE CENTER, PISCATAWAY, N.J., US, vol. 25, no. 2, 1 May 2010 (2010 May 1), pages 809-820, XP011297020, ISSN: 0885-8950, discloses VSC-based power-synchronisation control. The power control error is converted to a frequency deviation, which is then integrated to an angle increment. The output signal supplies the angle to transform the voltage reference from the converter dq frame to the stationary frame.
ZHANG WEIYI ET AL: “Frequency support characteristics of grid-interactive power converters based on the synchronous power controller”, IET RENEWABLE POWER GENERATION, THE INSTITUTION OF ENGINEERING AND TECHNOLOGY, MICHAEL FARADAY HOUSE, SIX HILLS WAY, STEVENAGE, HERTS. SG1 2AY, UK, vol. 11, no. 4, 15 Mar. 2017 (2017 Mar. 15), pages 470-479, XP006061208, ISSN: 1752-1416, DOI: 10.1049/IET-RPG.2016.0557 discloses a synchronous active power control solution for grid-interactive converters, as a way to emulate synchronous generators for inertia characteristics and load sharing.